Optical model based on transfer matrix method is employed to investigate the effects of active layer thickness and the configuration on the performance of organic solar cells based on P3HTPCBM. Simulation result reveals that short circuit current density increases with active layer thickness and performance of inverted structure device is superior to normal one. Interference effect has a great influence on optical electric-field strength distribution and the device performance. The short-circuit current density and optical electronic field distribution in two structures of the device are calculated to decide the optimal device configuration. It is found that the inverted organic solar cell device with 225 nm thick P3HTPCBM shows the highest performance allowing achievement of short-circuit current density for 15.5 mA/cm2, efficiency for 5.77%.